A generally known method of producing microfine frozen particles, comprises atomizing the material to be frozen, such as water, into an insulated vessel containing a refrigerant, such as liquid nitrogen, so that the atomized particles of the material to be frozen, as they sink into the refrigerant, freeze by heat exchange with the refrigerant. In such a method, however, it is difficult to collect the frozen particles that have piled up in the refrigerant.
In an attempt to solve this problem Japanese Patent Publication No. 58-17392 proposes a method wherein the freezing of the liquid particles takes place, not in a refrigerant but, in the cold gas phase of the vaporized refrigerant. According to this method, there is provided a freezing vessel wherein a nozzle for atomizing the material to be frozen is placed at an upper position, a pair of tubes for spouting or spraying a refrigerant inwardly are placed on the interior wall of the vessel and near and below the nozzle, one tube higher than the other, and a scraper is placed at the bottom of the vessel. In practice a liquid material to be frozen, such as water, is atomized downwardly from the nozzle while a refrigerant, such as liquid nitrogen, is sprayed in a mist inwardly from the tubes. The atomized mist of the material to be frozen comes in contact with the sprayed refrigerant and its vaporized gas, some in flows that cross each other, some in parallel flows, and others in opposing flows. As the mist falls down it is frozen by heat exchange. The frozen particles pile up at the bottom of the vessel and are collected from the vessel by means of a scraper.
In a vessel designed as above, when the gasification of the refrigerant lowers the temperature of the cold gas phase to a certain degree or lower (for example, -60.degree. C. or lower where liquid nitrogen is used as the refrigerant), it becomes possible for the sprayed refrigerant to fall to the bottom of the vessel in the liquid state without being vaporized. Where such a disorder is possible, it is necessary that the spraying tubes in the pair to be placed closer to the nozzle so that the sprayed refrigerant does not lower below a certain degree the temperature of its cold gas phase in the region within the reach of the sprayed liquid refrigerant. That is to say, the freezing vessel must be so designed that the particles, immediately after the atomization, pass through the region within the reach of the sprayed liquid refrigerant so as to accelerate the vaporization of the sprayed refrigerant by heat exchange with the particles from the atomization.
However, the contact of the atomized particles with the sprayed liquid refrigerant that takes place immediately after the atomization in a vessel arranged as described above makes it difficult for the particles to assume uniform globular shapes when frozen, because the, particles encounter the refrigerant before they assume globular shapes by surface tension. Deformed shapes, such as oval shapes, unavoidably result from such atomization. Moreover, agglomeration between the atomized particles is likely to occur as a result of their encountering the refrigerant and being frozen together and thus counteracting the effort to obtain frozen particles with uniform particle sizes. Needless to say, the frozen particles thus obtained are less than satisfactory as an abrasive for blasting, cleaning, etc.